Phosphazenes and their preparation are disclosed in U.S. Pat. No. 2,109,491. Partially substituted materials are mentioned in U.S. Pat. No. 3,206,494. p-Bromophenoxyphosphazenes are known, Dell et al, J. Chem. Soc. 4070 (1965). High molecular weight poly)-phenoxyphosphazene) has been described; Kugel et al, Inorganic Chem. 5, 1709 (1966).
Phosphonitrilic halides can be prepared by reacting NH.sub.4 Cl with PCl.sub.5 ; Emsley et al, J. Chem. Soc. (A), 768 (1971). Other references dealing with this preparation are cited therein and mentioned below.
Phosphonitrilic polymers containing P--O--P bonds are disclosed on pages 97-98 of Chemical Week, Feb. 20, 1965. Such polymers are also disclosed in U.S. Pat. No. 3,459,838.
Fire retardant use of phosphazenes with P--O--P bonds, and which also contain aryloxy groups, is disclosed in German 2,306,510. The compounds are made by reacting a phosphonitrilate polymer with phosphonitrilic chloride, or PCl.sub.3, POCl.sub.3 or PSCl.sub.3.
Various fire retardants have been suggested for polyesters; confer for example, U.S. Pat. Nos. 2,909,501, 3,285,995, 3,309,425, 3,434,981, 3,794,617 and Canadian 924,731.